Thermal printer

Abstract

To provide a thermal printer which can prevent the lifting phenomenon of the platen roller without reducing the printing speed. There is provided a thermal printer comprising: a frame ( 1 ) having a pair of side wall portions ( 2 a and 2 b ) arranged to face each other at a predetermined interval in a paper width direction; a head support member ( 4 ) for holding a thermal head ( 3 ); a platen roller attaching/detaching mechanism (A) for detachably and rotatably bearing a platen roller ( 10 ) by the above-described pair of side wall portions; a pressing mechanism (B, coil springs S 1 and S 2 ) for bringing a surface of the above-described thermal head into contact with a circumferential surface of the above-described platen roller; and a gear transmission mechanism (G 1 ) for transmitting a rotational driving force of a motor (M) to a driven gear (G 2 ) fixed at one end of the above-described platen roller, wherein pressing force applied on a shaft end on the driven gear side of the above-described platen roller by the above-described pressing mechanism is made to be within a range of 1.3 to 3.0 times of pressing force applied on the other shaft end.

What is claimed is:  
     
       1. A thermal printer comprising: 
       a frame having a pair of side wall portions arranged to face each other at a predetermined interval in a paper width direction;  
       a head support member for holding a thermal head;  
       a platen roller attaching/detaching mechanism for detachably and rotatably bearing a platen roller by the pair of side wall portions;  
       a pressing mechanism for bringing a surface of the thermal head into contact with a circumferential surface of the platen roller with predetermined pressing force so that pressing force applied on a shaft end on a driven gear side of the platen roller is made to be within a range of 1.3 to 3.0 times of pressing force applied on the other shaft end; and  
       a gear transmission mechanism for transmitting a rotational driving force of a motor to the driven gear fixed at one end of the platen roller.  
     
     
       2. A thermal printer according to  claim 1 ; wherein the average of the total pressure to be applied onto the circumferential surface of the platen roller by the pressing mechanism is in the range of 15 to 35 g/mm. 
     
     
       3. A thermal printer according to  claim 1 ; wherein the pressing mechanism is formed integrally with the platen roller attaching/detaching mechanism, the platen roller attaching/detaching mechanism is provided with a swing member having a U-shape as a whole and is slightly wider than the thermal head, the swing member comprises a pair of hook-like latch portions to be engaged with both shaft ends of the platen roller so as to draw the platen roller toward the surface of the thermal head, a pair of arm portions for guiding the latch portions along both sides of the thermal head to the rear side of the thermal head, and an elastic member support portion provided on its both sides with the arm portions and arranged in a width direction in the rear of the head support member for supporting elastic members, the swing member and the head support member are swingably supported through a support shaft provided between the side wall portions, an engagement release means for moving the swing member to thereby release the engagement between the latch portions and the platen roller is provided in the swing member, and one or two or more elastic members adapted such that pressing force applied thereby on a shaft end on the driven gear side of the platen roller becomes 1.3 to 3.0 times larger than that applied on the other shaft end may be held between the elastic member support portion of the swing member and the head support member. 
     
     
       4. A thermal printer according to  claim 3 ; wherein two or more the elastic members are used and each of the elastic members is arranged and positioned such that a barycenter of the pressing force applied on the platen roller is deflected toward the shaft end on the driven gear side of the platen roller by 7 to 25% whereby the pressing force applied on the shaft end on the driven gear side of the above-described platen roller may be 1.3 to 3.0 times larger than that applied on the other shaft end. 
     
     
       5. A thermal printer according to  claim 3 ; wherein in the case where two or more said elastic members are used, elastic members having the same elasticity are used and the compression amounts of the respective elastic members are varied so that the pressing force applied on the shaft end on the driven gear side of the platen roller may be 1.3 to 3.0 times larger than that applied on the other shaft end. 
     
     
       6. A thermal printer according to  claim 3 ; wherein in the case where the elastic members having different elasticity are used as the elastic members, each elastic member is arranged at the equal distance from the center, whereby due to the difference in the elasticity, the pressing force applied on the shaft end on the driven gear side of the platen roller that may be 1.3 to 3.0 times larger than that applied on the other shaft end. 
     
     
       7. A thermal printer according to  claim 3 ; wherein 
       the elastic member is made of a spring in general, rubber, synthetic resin or the like.